pgsql: Refactor script execution state machine in pgbench.

Refactor script execution state machine in pgbench.

The doCustom() function had grown into quite a mess. Rewrite it, in a more
explicit state machine style, for readability.

This also fixes one minor bug: if a script consisted entirely of meta
commands, doCustom() never returned to the caller, so progress reports
with the -P option were not printed. I don't want to backpatch this
refactoring, and the bug is quite insignificant, so only commit this to
master, and leave the bug unfixed in back-branches.

Review and original bug report by Fabien Coelho.

Discussion: <alpine.DEB.2.20.1607090850120.3412@sto>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/12788ae49e1933f463bc59a6efe46c4a01701b76

Modified Files
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src/bin/pgbench/pgbench.c | 1106 +++++++++++++++++++++++++++------------------
1 file changed, 661 insertions(+), 445 deletions(-)